One very important test for determining the condition of a producing oil well is a bottom hole pressure build up test. The results of this test indicate the need for well stimulation, work over, or recompletion, as well as permit the determination of formation characteristics. Occasionally, pressure sensors can be placed directly at the formation level within the borehole for direct measurement of pressure. However, more frequently, the presence of pumping rods in the tubing prevents such direct measurement. In those situations, it is common to use acoustic techniques to determine the level of the fluid within the borehole, and calculate the bottom hole pressure estimating the density and depth of the fluid column and overlying gas.
A typical pressure build up test will require three to four days to complete. Sufficient data points must be taken during this period to accurately represent the trend in pressure behavior during this period. With conventional acoustic techniques, each data point requires the involvement of personnel, thus creating a situation where such test can become prohibitively expensive.
U.S. Pat. No. 4,318,298, issued Mar. 9, 1982, U.S. Pat. No. 4,318,674, issued Mar. 9, 1982 and U.S. Pat. No. 4,389,164, issued June 21,1983 are each assigned to Mobil Oil Corporation, and disclose a technique which attempts to provide an automatic liquid level monitor and controller. This technique requires the fluid level to be pumped down to the pump inlet in the well at a known depth. A sonic level sensor is calibrated to this known depth. However, it is often impractical to pump a well to the pump inlet, and this technique does not account for variation in sonic velocity within the well after calibration.
A need exists for an improved apparatus and method for conducting acoustic pressure build up test. The cost of conducting such a test should be minimized, while the accuracy of the test is further enhanced and refined.